By micro-optics is meant here a field of optics where the operation of the optical elements used to guide light is not only based on macroscopic geometries of the element surfaces and thus on the overall three-dimensional shapes thereof but, first of all, on microscopic gratings extending on the element surfaces. When the optical performance is primarily determined by the grating structure, the overall shape of the element can be simple. For example, the element surfaces do not need to be curved like in conventional lenses but the grating can lie on a flat surface. This allows implementing the element bodies e.g. as flat plates with low thickness. Actually, the micro-optical elements are usually manufactured as thin film structures.
Micro-optics meant here covers both refractively operating grating structures and light manipulation by means of diffractive gratings. Diffractive optics forms a particular field of micro-optics. In diffractive optics, the operation of the optical elements are no more based on refraction of light ray at an interface between two materials having different optical densities as is the case in the refractive optics but, instead, on diffraction of light. In diffractive optical elements, the size of the structural details of the grating is in or below the magnitude of the wave-length of light.
Micro-optics is increasingly expanding into different application areas. One of its advantages is that the performance characteristics of the optical elements are variably and flexibly adjustable. On the other hand, development of the manufacturing techniques enables nowadays cost-effective mass production of micro-optical elements by different replication techniques. The general principle in these replication methods is that an inverse copy of the desired grating comprising micrometer-scaled or smaller three dimensional structures is provided in a particular mold/pressing tool which is used to press the grating structure on the surface of a suitable grating material.
As stated above, micro-optical elements are often manufactured as thin film structures. In a typical approach, a layer of liquid material, for example a suitable thermosetting resin, which is curable by ultraviolet (UV) light or by heat, is spread on a thin support/base film. The pressing tool is pressed on the thin film comprising the layer of the curable material and UV radiation or heat is directed simultaneously to the pressing point to cure the grating material. Because the dimensions of an individual micro-optical element are typically on the order of a few millimeters, it is possible to press by such a method a large number of elements in one film from which the individual elements are later cut off.
One of the most useful practical applications of the manufacturing principles presented above is a UV roll-to-roll process. In this approach, the pressing tool is arranged on the surface of a cylindrical roll/rod which rotates about its longitudinal axis, and the pressing takes place as a continuous process on a film which is led past the roll. The base film is coated by a UV curable resin, and UV radiation is directed to the point of contact of the pressing roll and the film.
The mass production methods described above are especially suitable for applications which require low cost micro-optical elements, which is the case for example in different consumer products. A typical example is a plastic flashlight lens of a digital camera or of an integrated camera element of e.g. a mobile phone.
Requirements for the material properties of the micro-optical elements are set primarily by the desired optical performance of the element. For example, a lens designed for visible light should have a high transmittance at the visible light wavelengths. Transmittance should also be uniform over said wavelength range. In other words, the lens should be transparent and optically clear without any coloring effect. This is usually the case also in spectroscopic applications where different wavelengths are specifically tried to be controlled in different manners. Differentiation between the wavelengths is realized by means of the actual grating structure, the performance of which is specifically dependent on the wavelength, the material itself being colorless.
However, particularly in consumer products, the appearance of all the visible parts of a device is also becoming increasingly important. In other words, besides its optical performance, it would be advantageous if a flash lens of a camera in portable devices of said type matched in appearance to the general appearance of the device cover. For example, it would be desirable if the lens could be tinted to match the general coloring of the device. Coloring of the optical element is not only relevant in flashlight arrangements but also applies to many other lighting arrangements. On the other hand, for example in mobile phones which are particularly critical in their appearance, it can be considered as something of a flaw or a disturbing factor if the LED element producing the flashlight shows through the lens. Suitable coloring of the lens could blot out the LED so as to be less noticeable.
Accordingly, there is a definite need of colored micro-optical elements and lighting arrangements utilizing such elements. However, the coloredness of the element should not affect substantially the primarily intended optical function of the element. Moreover, it should be possible to implement the coloredness of the micro-optical element cost-effectively as part of the normal mass production process of the elements.